However I'm having an odd problem with one of them. The program is called
Surtach3.exe & when run (in XP) brings up an error. However if I rename it
to Surtach3a.exe it runs happily!

The error message is not very informative though it refers to NT.DLL.

An earlier version of Surtach had several bugs, but this was uninstalled
before the new version was loaded. I saw there were still references in the
registry to the previous version, however, so ran Regclean which removed the
redundant entries. Irritatingly the problem persists.

I can get round it, but would like to understand what is happening.

Has anyone any suggestions?